Electrolytic method of cleaning containers



oxide coating.

United I States P n ELECTROLYTIC METHOD OF CLEANING CONTAINERS Ira J. Duncan, Detroit, Mich., assignor to Detrex Corporation, Detroit, Mich., a corporation of Michigan No Drawing. Application July 20, 1953, Serial No. 369,253

4 Claims. (Cl. 204-141) This invention relates to a method of cleaning containers, and more specifically concerns an electrolytic method of cleaning carbonized and grease-carbon incrustations from baking pans without corroding the pans. More specifically, the invention concerns the application of an alkaline cleaning solution to oneor more breadbaking pans wherein at least two unlike metals or metal oxides are immersed in the solution.

Most pans used in the bread baking industry are made of tin coated steel and have reinforcing bands constructed of heavier tin coated steel or galvanized steel. Recently, aluminized steel bands have been used by the pan manufacturers on tin coated steel pans. Such pans are usually subjected to a heat treatment of about 425 F. for several hours prior to being used. In the opinion of the trade, superior baking characteristics are thus obtained. This treatment gives to the pans a grayish blue color, commonly referred to as burn-in, or tin oxide coating. Accordingly the pans may. have tin or tin oxide surfaces, while the straps may have tin, tin oxide, aluminum, or aluminum oxide surfaces.

it has heretofore been common practice to clean soiled tin-plated steel bread pans by immersion in a boiling alkaline cleaning solution in a boil tank. It has been necessary to inhibit such cleaners in order to prevent attack on the tin oxide surface of the pan. This has usually been accomplished by the addition of chromate and/0r silicate salts to the cleaner, as disclosed in the patents to Adler No. 1,734,706 and Scott No. 1,879,953, for example. But when cleaning pans constructed of both tin and aluminum, or when amixture of tin and aluminum pans is cleaned in a common solution, the above mentioned inhibited cleaners do not protect the tin surface adequately.

When two different metals are brought into contact with the cleaning solution an electrochemical phenomenon is involved. In the case of tin coated steel pans bearing an oxide film and covered with carbonized grease and other bakery soils, there is areducing action on the Thereby, numerous small areas of the oxide film are often removed which exposes the pure metal to the solution at these points. The more noble metal oxide and the pure metal in contact with the cleaning solution tend to set up a galvanic cell action. Also, when two metals, e. g.,-tin and aluminum, are in contact with a common cleaning solution there is a tendency for both tin and aluminum to go into solution. When this takes place the metals become electro-negative. Since the solution potential of tin and aluminum are not of the same magnitude there is set up a galvanic action between the two metals. Pans constructed of both tin and aluminum and covered with bakery soil might exhibit an even greater galvanic action than similar clean pans.

-' The function'nof inhibitors. is to passivate the metal surface and thus prevent the oxide from being removed.

2,778,791 Patented Jan. 2 7

ice

and to prevent the galvanic cell activity. This is sometimes difiicult to accomplish completely with certain types of bakery soil and with certain types of pans. Galvanic cell activity as outlined above interferes with the inhibiting action of chromate or silicate inhibitors.

It is accordingly an object of this invention to provide a method which overcomes the foregoing difliculties and disadvantages. Another object is to provide a method of cleaning carbonized and grease-carbon in crustations from baking containers having surfaces containing at least two materials selected from the groupconsisting of tin, aluminum, tin oxide, and aluminum oxide, without appreciable attack on the surface. Other objects and advantages of the invention will further appear hereinafter.

I have discovered that by applying an outside potential to the metal surface as great as, or greater than the potential difference between the metals present and between the metals and their oxides, all such pans can be effectively protected from attack while cleaning in chromateand silicate-inhibited alkaline cleaners. This is true for any type of soiled tinned pans or soiled combination tinned and aluminum pans. This process is also effective in certain solutions containing chromates only or silicates only as inhibitors. Tin pans can be successfully cleaned in the usual alkaline cleaners when they are inhibited with chromates only. The presence of silicates gives further protection to the tin surface. However, in many instances the cleaner may be free of silicates, with protection provided by the electrolytic method of this invention. When two different metals are prescut, as in cleaning tin pans provided with aluminized straps or cleaning mixed loads of tin and aluminum pans, the silicates are necessary components of the composition in order to protect the aluminum surfaces.

In practice the cleaning is preferably performed in a boil tank containing a boiling solution of the alkali cleaner referred to above. The soiled baking pans are preferably loaded into a basket made of metal or other electric conductor. The positive side of a direct current source, such as a rectifier or battery charger, for example, is connected to the basket which is the anode. The negative side of the direct current source is connected to a small rod or other contact disposed in the solution which serves as the cathode. The basket is lowered into the solution in the boil tank and a direct current is passed through the solution while the solution is maintained at boiling temperature. The potential drop is adjusted to such a point that the current density at the anode is maintained preferably in the range of one to five amperes per square foot. It is particularly desirable to maintain the current density at about l5 amperes per square foot. A typical potential drop under these conditions is l-8 volts.

The cleaning composition preferably contains a substantial proportion of alkali metal carbonate as well as water-soluble silicate and chromate. It may also contain soluble phosphate.

Among the silicates that serve as inhibitors are anhydrous sodium silicate, hydrated sodium metasilicate and anhydrous or hydrated potassium metasilicates and the other alkali metal metasilicates and their equivalents.

Among the chromates that serve as inhibitors and coact with the above identified silicates are the chromates and dichromates preferably of the alkali metals such as sodium, ammonium or potassium and their equivalents.

The cleaning solution preferably comprises from about 2 to 8 ounces of total cleaner per gallon of water, for example 5 ounces per gallon.

. The preferred cleaners in accordance with, this invention contain the following chemicals in the ranges of weight proportions indicated:

Percent Soluble phosphate or polyphosphate (expressed as PO4=) 3 to 15 Soluble chromate (expressed as CIO4:) 3 to 8 Soluble silicate (expressed as SiOs 6 to 45 Alkali metal carbonate (expressed as COa 15 to 50 The following examples further indicate preferred procedures for practicing the invention:

Example 1 A solution was prepared consisting of the following materials in water:

Sodium meta silicate 10.45 Trisodium phosphate 22.50 Soda ash 63.00

Sodium chromate u The solution contained 4 ounces of the above chemicals per gallon of water. Soiled tin bakery pans were cleaned in a boiling solution of the above composition in which a voltage of 4 volts and a total current density of 3 amperes per square foot were maintained with the pans anodic. Cleaning was complete after a period of 3 hours. The pans were successfully cleaned without damage to the tin surface.

Example 2 Percent Sodium meta silicate 15 Soda ash 85 A solution of the above composition at ounces per gallon, operated at 4 volts and 1 ampere per square foot current density cleaned sections of soiled aluminized steel-tin pans without attack on tin or aluminum in 5 hours. The pans were anodic. The same solution brightened tin in minutes without this electrolytic protection.

Example 3 Percent Sodium meta silicate Sodium tripolyphosphate 10 Soda ash 70 Sections of soiled aluminized steel-tin pans were cleaned in an aqueous solution of the above composition containing 5 ounces per gallon of water without attack on the tin or aluminum surfaces when a current density of 1 ampere per square foot and a voltage of 3 volts were supplied with the pans anodic.

Example 4 Percent Trisodiurn phosphate Soda ash 40 Sodium chromate 10 Sections of tin pans were cleaned in the above composition at a concentration in water of 4 ounces per gallon when 3 volts and a current density of 2 amperes per square foot were applied with the pans anodic, without attack on the tin surface.

Example 5 Percent Potassum meta silicate 72.5 Disodium phosphate 27.5

tained without any appreciable pan corrosion.

Having thus described my invention, I claim: 1. In the method of cleaning an object having a surface containing at least two dissimilar members respectiv'ely selected from each of the following groups: (1) tin and tin oxide and (2) aluminum and aluminum oxide, which dissimilar members tend to enter into galvanic corrosive activity in the presence of strongly alkaline electrolyte whereby said electrolyte attacks the tin or tin oxide, the novel method of inhibiting such corrosive activity without interfering with said cleaning comprising placing said object in a hot aqueous alkali cleaner solution and passing through said surface and through said solution a direct current having a potential of about 1-8 volts and at least equal to the potential difference between said two dissimilar members, said surface being connected as an anode.

2. In a method of cleaning baking containers containing at least two dissimilar members respectively selee ad from each of the following groups: (1) tin and tin oxide and (2) aluminum and aluminum oxide, said surfaces also having carbonized incrustations thereon, which dissimilar members tend to enter into galvanic corrosive activity in the presence of strongly alkaline electrolyte whereby said electrolyte attacks the tin or tin oxide, the novel method of inhibiting such corrosive activity without interfering with said cleaning comprising placing said baking containers in a boiling alkali cleaner solution containing an alkaline inhibitor selected from the group consisting of the alkali metal chromates and silicates, passing through said containers and said solution a direct current with the containers as anodes, and maintaining the potential drop across the solution at a value to produce a current density of 1-5 amperes per square foot.

3. A method of cleaning incrustations from bake pans comprising placing the pans in a metallic container, immersing the pans and container in a boiling alkaline cleaning solution containing substantial proportions of chromate and silicate ions, said pans and container together providing dissimilar members including both (1) coating selected from the group consisting of tin and tin oxide and (2) coating selected from the group consisting of aluminum and aluminum oxide, both (1) and (2) being immersed concurrently in the bath, which dissimilar members tend to enter into galvanic corrosive activity in the presence of strongly alkaline electrolyte whereby said electrolyte tends to attack the tin or tin oxide, and passing through said pans, container and solution a direct current with the containers and pans as anodes, and maintaining the potential drop across the solution at a value of about l-S volts and at a value at least substantially equal to the potential ditference between said dissimilar members.

4. A method of cleaning carbon-grease food incrustations from surfaces containing at least two dissimilar members: (1) selected from the group consisting of tin and tin oxide, and (2) selected from the group consisting of aluminum and aluminum oxide, comprising immersing said surfaces concurrently in a solution of a composition consisting essentially by weight of about 3-15 soluble salt selected from the group consisting of phosphate and polyphosphate salts, about 3-8% soluble chromate, about 645% soluble silicate, and about l5-50% soluble carbonate, passing a direct current through said surface and solution with the surface as anode, and regulating said current to provide a potential drop of about l-8 volts and an anodic current density of about 1-5 amperes per square foot.

References Cited in the tile of this patent UNITED STATES PATENTS 2,312,076 Cook et a1. Feb. 23, 1943 2,606,866 Neish Aug. 12, 1952 FOREIGN PATENTS 102,392 Australia Nov. 11, 1937 OTHER REFERENCES The Cleaning of Metal, Mitchell (1930), p. 49 cited. Published by Magnns Chem. Co., Garwood, N. Y. 

2. IN A METHOD OF CLEANING BAKING CONTAINERS CONTAINING AT LEAST TWO DISSIMILAR MEMBERS RESPECTIVELY SELECTED FROM EACH OF THE FOLLOWING GROUPS: (1) TIN AND TIN OXIDE AND (2) ALUMINUM AND ALUMINUM OXIDE, SAID SURFACES ALSO HAVING CARBONIZED INCRUSTATIONS THEREON, WHICH DISSIMILAR MEMBERS TEND TO ENTER INTO GALVANIC CORROSIVE ACTIVITY IN THE PRESENCE OF STRONGLY ALKALINE ELECTROLYTE WHEREBY SAID ELECTROLYTE ATTACKS THE TIN OR TIN OXIDE, THE NOVEL METHOD OF INHIBITING SUCH CORROSIVE ACTIVITY WITHOUT INTERFERRING WITH SAID CLEANING COMPRISING PLACING SAID BAKING CONTAINERS IN A BOILING ALKALI CLEANER SOLUTION CONTAINING AN ALKALINE INHIBITOR SELECTED FROM THE GROUP CONSISTING OF THE ALKALI METAL CHROMATES AND SILICATES, PASSING THROUGH SAID CONTAINERS AND SAID SOLUTION A DIRECT CURRENT WITH THE CONTAINERS AS ANODES, AND MAINTAINING THE POTENTIAL DROP ACROSS THE SOLUTION AT A VALUE TO PRODUCE A CURRENT DINSITY OF 1-5 AMPERES PER SQUARE FOOT. 